The present invention pertains to a device for use with a compound bow having eccentric wheels at the ends of the upper and lower limbs, around which the bowstring cables are trained. The compound bow is shown and described in U.S. Pat. No. 3,486,495 to H. W. Allen, to which reference may be had. The eccentric wheels each rotate approximately 180.degree. as the bow is drawn to full draw, and these eccentric wheels, together with a block and tackle arrangement of the bowstring cables, are responsible for a unique characteristic of the compound bow whereby the draw weight of the bowstring increases rapidly to a maximum at about 78% of full draw distance, and then decreases about 20% as full draw is approached. For maximum speed and accuracy, it is essential that the upper and lower eccentric wheels rotate in exact synchronism, so that they peak and turn over at the same time. It is difficult to determine whether the eccentric wheels are turning in synchronism, as they are at opposite ends of the bow, and it is necessary to shift the eyes first to one end of the bow and then to the other. It is virtually impossible to tell by looking from one eccentric wheel to the other when they are turning in synchronism, and it has heretofore been a very tedious and time-consuming operation to get the eccentric wheels of a compound bow to peak and turn over at the same time.